A study was conducted to explore the nature and effectiveness of peer feedback in EFL writing classrooms. High- and low-proficient Japanese EFL learners participated in the study where they exchanged comments with peers after receiving training in peer feedback. The two groups were compared in terms of their pre- and post-tests, original drafts and rewrites, peer comments and responses to the comments. It was found that peer feedback had overall positive effects on the compositions for both the high- and low-proficient students, with different patterns observed in the relationship between the comments and revisions that characterized the two groups.